This invention relates to a method for grading up the dielectric breakdown strength of a glass neck tube of a cathode ray tube (CRT).
It has been known that the dielectric breakdown strength of a dielectric or insulator material, such as glass, varies correspondingly to various conditions at a time when a high voltage is applied to the material. The edge effect of an electrode mounted on the dielectric material is one of the functions that remarkably degrade the dielectric breakdown strength of the dielectric material. The edge effect is thought to be based on the fact that the electric field concentrates on the edge of the electrode.
As well known in the art, a CRT has an internal electric conductive film of graphite or an aquadag, which is a mixture of graphite fine powder and water glass, which is applied on the internal side surface of a glass funnel of the CRT and extends to an internal surface of a glass neck tube to terminate an edge thereof.
In the neck tube, an electron gun is contained, and one of electrodes of the gun is electrically connected to the internal electric conductive film through metal springs which support the gun within the neck tube. The other electrodes of the gun are connected to stem pins which are mounted on an end of the neck tube for electrical connection with a peripheral circuit. The internal electric conductive film is electrically connected to an anode terminal which is mounted in the side wall of the funnel.
In use, a D.C. high voltage for example, 30 KV, is applied to one of electrode of the gun through the internal electric conductive film and the anode terminal.
It is desirable to elevate the D.C. high voltage applied to the anode terminal in order to improve the brightness of the image developed on the viewing screen of the CRT.
But it is experientially known that the higher the D.C. voltage applied to the anode terminal is, the more often dielectric breakdown of the neck tube is caused near the edge of the internal electric conductive film.
When the outside surface of the neck tube is electrically grounded, the concentration of the electric field at the edge of the internal electric conductive film is promoted so that the dielectric breakdown of the neck tube may be caused in a direction of the thickness of the glass wall of the neck tube.
While in use in a high humidity condition, the outside surface of the neck tube of the CRT may be effectively brought into a grounded condition. In a TV set having an electronic channel selector, it is desired that the outside surface of the neck tube is electrically grounded in order to ensure the unerring operation of the selector.
Accordingly, it is desired to grade up the dielectric breakdown strength of the glass neck tube of the CRT.